General Physics Thread

Hmm, in order for a planet to be "stuck halfway into a wormhole", I think the WH has to be larger than the planet. This implies that it would be impossible to be in orbit around this structure without noticing it (it may not even be possible to be in stable orbit around it, I'm not sure). You could perhaps put the WH inside the mountain and keep it hidden from casual observation that way.

A wormhole small enough to fit into a mountain will produce some staggering tidal effects on its inside, 'yes saying.

Would it? A WH isn't a black hole, IIRC. If the mass is equal on both side of the mouth, why would there be tidal effects?

The spacetime distortion from a wormhole is just as significant as from an equivalently-sized black hole, actually.

When you say "size" do you really mean mass?

Last I checked, the mathematical formulas are quite different - black hole size scales with mass, wormhole size with the inverse square (root?) of the negative energy density.

Currently reading: Euclid's Window ^note  Full title: Euclid's Window: The Story of Geometry from Parallel Lines to Hyperspace, by Leonard Mlodinow.

As the title indicates, the book is mostly about history of geometry and how it influenced beyond math and science throughout human civilization.

The book is divided in five parts, each focusing on different mathematician/scientist (in chronological order): Euclid, Descartes, Gauss, Einstein, and Edward Witten. Just finished part 1 and halfway through 2.

And I gotta be honest...I never even heard of Witten until opening this book. So naturally, I was quite surprised by the sheer width of his ideas and influences, such as string theory and quantum gravity, M-theory, etc.

Just goes to show how absolutely little I know about physics and the field's pioneers, I guess.

Edited by dRoy on Aug 30th 2023 at 9:13:41 PM

"Physics Twitter" is currently in a massive, although mostly cordial debate about whether gravity is a real or pseudo force. Examples of the latter include the centrifugal and Coriolis forces, which derive solely from the inertial behavior of objects, but look like forces in certain reference frames.

As I described it, human beings aren't evolved to understand inertial reference frames because we don't live in one. On Earth, we are under a more-or-less constant 1 g of acceleration, but that acceleration isn't because we're falling; rather it's due to the matter beneath us resisting gravity via electrostatic repulsion. We feel gravity because the ground keeps us from falling.

If you've ever ridden a roller coaster or fallen a long distance, you know the actual feeling of free-fall, which we instinctively find terrifying because it means we're about to come to a very painful stop. (It also messes with our vestibular system, which coordinates balance.)

Gravity is the warping of spacetime due to mass-energy. Note that it's important to include the -time portion of spacetime, because without it, nothing would ever fall (or move at all, but I digress). Specifically, gravity represents a curvature of the future (and past) path you follow through space, causing otherwise parallel vectors to converge or diverge.

An object undergoing no acceleration will follow that path, called a geodesic, forever. However, matter doesn't like geodesics that intersect; these lead to collisions because the particles can't occupy the same space at the same time.

There are four basic forces that act against gravity:

• Electrostatic repulsion. This is the dominant force in "conventional" reference frames, like the surface of a planet. The electrons in atoms don't like being forced together because they all have negative charges. Thus, they repel one another with a strength inversely proportional to the square of the distance between them. You don't fall through the floor because the atoms in the floor repel the atoms in your feet. Your body doesn't collapse into a puddle of goo because the atoms in your cells repel each other. Electrostatic repulsion also increases with temperature; this is why hotter things puff up and why gases want to expand while solids don't.

• Photon pressure. This is the dominant force in the cores of stars. The energy released by nuclear fusion props them up against the immense crush of gravity. (It's related to electrostatic repulsion because both are caused by the electromagnetic field.) When this force goes away, such as in the cores of dying stars, they stop resisting gravity and collapse, giving rise to...

• Electron degeneracy pressure. Electrons, being fermions, cannot simultaneously occupy the same quantum state, a phenomenon known as the Pauli exclusion principle. (This is why atoms have distinct electron orbital shells and is the foundation for all of chemistry.) If you try to force them closely together, they will first fill all available energy levels, down to the lowest, and then they will push back with incredible force. This is what supports the mass of a white dwarf, the remnant of a dead low-mass star, after its fusion fuel runs out. But a more massive star will use...

• Neutron degeneracy pressure. Once electron degeneracy is overcome, electrons are forced to merge with protons, becoming neutrons. At that point a stellar remnant collapses even further until an even stronger force takes over: neutron degeneracy. Neutrons are also fermions, and they resist being compressed much, much more powerfully than electrons. No known force can overcome this pressure, except for a hypothetical state called quark matter, in which neutrons break down into their constituent quarks.

The next step after a neutron star is a black hole, because the density of matter needed to overcome neutron degeneracy is also enough to pass the Schwartzchild limit.

To summarize, the "force" that we typically think of as gravity is in fact the electrons that make up ourselves and everything around us resisting their natural tendency to fall toward the Earth's center of mass.

Another reason why it's hard for humans to picture how gravity really works is because we're used to thinking of geometry as existing within a fixed set of axes with evenly spaced markings. The axes themselves don't change; they can't, or our concepts wouldn't make sense. How would you draw or measure a triangle if the distance between 1 and 2 were different than the distance between 2 and 3?

But this is exactly how gravity works. What we perceive as a force is actually the distortion of geometry that occurs when the distances between the markings on the graph aren't constant, and change over time.

Moreover, as I said earlier, one of the axes is time. That's even harder to describe intuitively. We are all flowing through time at a steady rate; in fact, the majority of our proper motion is through time, unless we're accelerating very rapidly. In these terms, an inertial object in its own reference frame is moving through time at precisely the speed of light.

Gravity changes the distances between points on each axis, and thus bends our natural path through spacetime. From a different reference frame, we are no longer moving straight and thus our clock evolves at a different rate. Non-inertial motion (Newtonian acceleration) also changes our path, with corresponding effects on our clocks.

Edited by Fighteer on Sep 30th 2023 at 2:21:24 PM

I guess it could be more like the three other forces affect the things in the sandbox/universe, while gravity affects the sandbox/universe itself?

Oh, that argument's back?

Well it's better than Joe Rogan trying to revive tired light for some reason, and has slightly better grounding.

I mean the grounding is still bad, the argument for gravity being a pseudoforce, as near as I can tell, is "it's not currently working with QM, so if we just say this thing with a field (spacetime) but no detected boson (because it's a very weak force and thus would have very hard to detect particles) is a pseudoforce then we can say gravitons, if they exist, are a pseudoparticle and go home."

It's a half-hearted resolution to the GR-QM conflict, I think though, just sort of saying "okay what if gravity just doesn't count as a force in the same way as the others and we don't have to worry about it as much." There's presumably a more accurate resolution.

Is there a debate to be had at all? My understanding is that gravity is an inertial force in GR (because inertial frames are free-falling), but outside of GR it's usually more convenient to take it as a real force.

And this has nothing to do with the GR-QM conflict, it's all GR.

Edited by Aetol on Sep 30th 2023 at 12:10:15 PM

Just like the centrifugal and Coriolis forces, gravity looks like a force in certain reference frames, hence we call it a pseudoforce. However, if you are falling in a gravitational field without any countering force, you are in an inertial reference frame.

Edited by Fighteer on Sep 30th 2023 at 9:14:46 AM

... wouldn't the fact that its bending space time make it a force any way, even if its not acting on the object itself, it is acting on reality still.

No, because spacetime has no mass-energy and thus momentum. You can't work with Newton's laws in this context.

But spacetime can expand and contract, and these movements can accelerate or slow down.

I thought inertial reference frames require an absence of acceleration (in any direction)? Or am I thinking of a different term again?

That's exactly the thing. In a gravitational field with no external forces, you are inertial. From your point of view, there is no acceleration. If you let go of a ball, it will remain where it is relative to you (absent tidal effects).

The path you trace through spacetime in an inertial reference frame is called a geodesic, and it follows the curve of gravity.

Edited by Fighteer on Sep 30th 2023 at 5:06:39 AM

But if an accelerating and non-accelerating reference frame interact with each other you can tell which one you're in based on relativistic effects, which isn't true for two non-accelerating reference frames with different velocities. I think. I'm feeling very hazy about this right now and it might be making me more confused.

The comparison was to the pseudoforces produced by rotating reference frames, which are not inertial because they're accelerating (perpendicularly to the direction of motion, rather than parallel as in freefall).

When spacetime can be considered a field, as near as I can tell, it doesn't seem right to consider gravity a pseudoforce because what we see as gravity is "only the warping of spacetime".

After all, electromagnetism has its own field that permeates everything, and that gets warped, but we don't consider electromagnetism a pseudoforce.

You should always be able to tell if you are in an inertial reference frame or a non-inertial one, because things behave differently in non-inertial frames.

The ball test is one such. If you let go of it, it will try to be inertial. If you are not, then it will have different motion.

What I think you're confusing is that it is not possible to tell non-inertial reference frames apart if they have equivalent acceleration vectors. The classic example of this goes as follows:

1. Enter a sealed, perfectly rigid box. You can't see, hear, or feel anything from outside. (It's in a vacuum and you can breathe somehow. This is a thought experiment.)
2. You cannot tell the difference between that box being on the ground on the surface of Earth and that box being in deep space with a rocket engine attached that is providing exactly 1 g of upward acceleration.

The obvious corollary to this is that, in that same box, you can't tell if you're floating in space or falling towards a gravitational body, since those are both inertial. But you can tell an inertial frame apart from a non-inertial one by letting go of a ball inside the box. It will obey "pseudoforces" like gravity, centrifugal, or Coriolis despite the fact that it feels no force in its own reference frame.

Tides are another pseudoforce experienced by objects under gravity when the curvature is substantially different across different parts of an object, but they give rise to real forces when they cause those objects to undergo mechanical stress.

That's actually one of the interesting debates in physics. Spacetime is not a field in quantum mechanics. Rather, it is a framework in which fields operate. Most quantum calculations assume locally flat spacetime; they get squirrely when significant curvature is introduced.

Similarly, gravity is not a field in the Standard Model. There's no place for it in the calculations. Many scientists think it must be a field, and have designated the graviton as its carrier particle, but no evidence of this has ever been found, and it doesn't work in the math of QFT.

Combined, these two factors create the great gap between general relativity and quantum mechanics that is waiting for us to solve some day. Many ideas, like string theory, could close that gap, but there is as yet no experimental proof for any of them.

Edited by Fighteer on Sep 30th 2023 at 6:13:14 AM

Ohhhhhhhh, got it got it got it. Because gravitational "charge" is the same as inertial mass/energy, unlike electromagnetic charge or quark colour.

...how is that argument affected by the existence of the Higgs field?

The Higgs field couples to other fields to give their elementary particles mass, yes. I'm not perfectly clear on this, but I believe that it works by inducing a kind of "drag" that prevents them from moving at the speed of light, and this is because the Higgs field has a non-zero rest state. But that's a little beyond my current layman's understanding.

However, you don't need Higgs mass to generate spacetime curvature. Energy alone is sufficient. Also, none of this depends on what gravity itself is.

I mean if gravity turns out to be several unrelated phenomena that the anthropic principle has bundled together in a semiotic trenchcoat I wouldn't be surprised. I've wondered the same thing about time. My thought process was that the existence of the Higgs field has implications about the differences between mass and energy that might make it possible to distinguish between gravity and inertia on the quantum scale. It's a little unclear to me whether inertia is well-defined as a force, or as a property of matter or of the fabric of the universe. Maybe inertia is an emergent property of gravitational fields.

These are good thoughts, but so far no experimentation has been able to distinguish gravitational from inertial mass. As far as we can tell, gravity just is. There's no way to uncouple it from spacetime itself.

The old bedsheet analogy can be used here. Take a bedsheet, stretch it taut, then drop something in it. The deformation in the sheet is conceptually similar to the curvature induced by gravity. If you roll a ball on the sheet, it won't go straight; it will instead tend to fall towards the center of the deformation. If the sheet were frictionless, the ball would orbit the mass.

The ball also deforms the sheet, creating its own little dimple that pulls the larger mass towards it.

Now take that bedsheet analogy and transform it into four dimensions. You can't, because our minds are incapable of picturing it, but that's more or less what gravity is. Gravity isn't part of any field; it's the background on which all the other fields operate. Spacetime curvature is not a force; it's a future path through three-dimensional space.

If we could figure out how to quantize gravity (i.e., describe it as a field), we would be able to unify general relativity and quantum mechanics, a holy grail of physics.

Edited by Fighteer on Oct 6th 2023 at 11:29:41 AM

Mathematically speaking, the Higgs field is like an electrical superconductor. Magnetic fields and electrical fields cannot exist in a superconductor, because it responds to them frictionlessly by creating an opposite field/conducting them away. Thus a magnetic field is compensated around the superconductor and so "squeezed out" ("Meissner effect"), which is mathematically equivalent to giving photons a mass. Electrical superconductors give mass to photons and the Higgs field to the W and Z bosons.